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Old 11-03-2015, 07:08 AM   #1
devenh
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Default Advantages of double clutching explained

When I went to the Bondurant Driving School a really, really long time ago, we were taught to double clutch.

Fast forward to 2012 when I join the club and attend my first PDX at RFR. I ask about double clutching and the universal answer seems to be it is no longer necessary with a modern manual transmission and better synchros. Okay, this makes sense (since it has been a really, really long time since I've been on track).

Since I never understood what was going on in the transmission, I had no basis to challenge these opinions. My lingering curiosity about what's actually going on got the best of me, and thanks to Youtube I found these videos:

The first one is for those who are not familiar with what is going on in the gearbox. Among other things, it explains what synchros do:

https://www.youtube.com/watch?v=vOo3TLgL0kM

Here is the important one. It explains why double clutching is useful on the track:

https://www.youtube.com/watch?v=zy0BfG_CG2Y

So let me know what you think? Do you double clutch now? If not, does this video change your mind?
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Old 11-03-2015, 09:07 AM   #2
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There's terminology changes that have taken place in the last ten years or so, that always leave me confused when people talk about this...

Double Clutch was something that was necessary in older vehicles, without synchros or with early rudimentary ones. In order to select another gear, often either up or down, you had to depress the clutch and pull the selector out of the current gear, then release the clutch pedal with the trans in neutral in order to match the speed of the input shaft to the main shaft. Then depress the pedal again and select your next gear. Thus the Double Clutch name: it required depressing the clutch TWICE to change gears.

What everyone on the internet these days calls double clutching was always simply known as Rev Matching. Using the throttle to match the engine speed to an estimation of the trans output speed in the lower gear before you engage the clutch on a downshift. It's all Rev Matching, whether you do it singularly, or the advanced 'heel and toe' style where you operate the throttle with the heel or side of your right foot, WHILE still braking into the corner.

Maybe I'm just 'old school' in my terminology...

That being clear where my definitions lie, I ABSOLUTELY Heel and Toe rev-match all of my downshifting on the track! In fact, 99 times out of a hundred, I do it when approaching any slow down or stop on the street, just to keep the practice up.

On track, the smooth engagement of the clutch helps maintain the vehicle's stability when you're already braking at the traction threshold, and rev matching the engine into the lower gear helps TREMENDOUSLY with that smooth engagement. I can tell you for sure, the Heel and Toe method takes quite a while to be any good at...! But it's worth it!
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Old 11-03-2015, 12:40 PM   #3
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Agree with Master Cory. If I am super lazy on the street, or for some reason cant get the car into a gear I want with out excessive force on the shift lever (with the clutch depressed) I will let the clutch out, press it in again to select the desired gear. Only on the street, or if something else is wrong. When competitively driving, I have never used or see an advantage to Double clutch. Heel toe, is an essential skill to a drivers arsenal, it is not optional, it is mandatory to master. Absolutely beautiful when done correctly at speed.
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Old 11-04-2015, 07:32 AM   #4
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Cory and Ed nailed it, especially about today's incorrect usage of the term double clutching. Double clutching was about taking the pressure off the gears on an upshift and speeding up the input shaft with a blip in neutral with the clutch out on a down shift. Hasn't been necessary since people started experimenting with syncros in the '40's and Porsche built the first trans with cone type syncros in the early '50's.

They're right about heel & toe being an essential skill. Cory said "Stability", absolutely.
Think about the way we teach the novice/intermediate drivers.
Brake-Downshift-Turn In , fine for drivers that are learning what a line in a corner is but once you start going fast:

Brake - you're loading the front tires and the front end of the chassis comes down , Downshift - at this point you come off the brake, the front of the chassis comes back up, your front tire contact patch decreases quite a bit which lowers the max corner entry speed and increases the chances of developing a push on corner turn in.

When you heel & toe, the front end stays down, the tire contact patch stays close to the same because you're still on the brakes. You can threshold brake for a longer distance (because downshifting without H&T is just coasting).

It also makes for a good transition into trail braking if the corner requires it.

Rev matching is also easier on the whole drive train.

As Cory & Ed said "Smooth is Fast"
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Old 11-05-2015, 10:29 AM   #5
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You'll find that once you get used to "heel/toe" you begin to do it around town without even thinking about it, just a muscle memory thing. It took me about a year of constant practice to get it right (and an add on pedal extension) with the Miata but now it's as natural as stepping on the clutch pedal to shift, and smooths out all the transitions.

I, too, am old enough to remember double clutching, and it is one of those lost skills we geezers have. Give me an ol' Corn-Binder pickup with a crash box and I'm still good to go though.
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Old 11-06-2015, 02:33 PM   #6
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I had to learn to double-clutch downshift thanks to autocrossing a WRX. A stock WRX transmission circa 2002 is nearly impossible to get to downshift into 1st gear quickly on the autocross course. But, adding in a double clutch during a rev-matched downshift allows you to skip the long synchro delay for getting into 1st on those really tight turns.

So, it's not a completely useless skill... it's useful for shifting around bad or faulty equipment.
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Old 11-07-2015, 07:23 AM   #7
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If I understand the videos correctly, rev matching still relies on the synchros to get the input shaft spinning at the same speed as the intermediate shaft.

The action of double clutching (with rev matching) speeds up the input shaft so that when the clutch is engaged the second time and the gear changed, there is less wear on the synchros because the input and intermediate shafts are much more closely matched in rotation speed than they would be when single clutched.

Sorry if any of my terminology is wrong.

Probably should have made it clearer that my use of the term double clutching also includes rev matching.

One of the points of the video was that while double clutching (with rev matching) was not needed for normal driving, it does prevent synchro wear under the more extreme conditions on the track.
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Old 11-08-2015, 06:32 AM   #8
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If your goal at a track event is to save your syncros by running slower lap times, go for it and double clutch.
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Old 11-08-2015, 10:49 AM   #9
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To expand on what Bill said, the saving advantage of a true double clutch shift is not worth the time and effort required, on the race track, in a modern car where it's not necessary due to the car's equipment. Whatever life is saved on the synchros would be negligible, and you'd hate what it did to your lap times...!
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Old 11-10-2015, 02:02 PM   #10
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It is true that Heel-toe does not save syncro wear. It keeps the chassis undisturbed during instances of threshold braking while down shifting. Double clutching does remove 99% of wear which would normally act on synchronizers within the transmission when used with a rev-match for down shifting. Technically the video you watched was correct in that. And all of the responses on this thread are also correct; no one uses this technique for competitive driving. Last person I saw double clutching was my school bus driver 20 years ago. He did this because the buses trans had no syncros. Maybe Deborde will chime in. Semi trucks and Buses don't have them because the transmissions are expected to last 1 million miles or more. Syncros would wear out well before that mark.
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Old 11-10-2015, 03:55 PM   #11
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Heel-toe and rev matching helped in the snow this morning
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Old 11-11-2015, 02:08 PM   #12
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Randy Probst talks about the dying art of shifting on p. 20 of the November issue of Sports Car and concludes "that double clutching is overkill."
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Old 12-04-2015, 08:29 PM   #13
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Interesting discussion and here is my 2 cents worth. Theoretically, double clutching the down shift doesn't generally make your lap times slower as you are in the process of slowing down under braking anyway and there is adequate time to get it all done. The exception might be if you just happen to find yourself in the wrong gear and you need a lower one NOW!

Going way back in time..........when I started racing, I didn't match revs on the down shift, resulting in huge broad slides and an overheated transmission. I forced myself to learn rev matching and experimented with double clutching too. I found that double clutching required too many brain cells and coordination chits for me to spread amongst my threshold braking adventures. I gave up on the whole double clutching thing and simply do the rev matching thing now. It is practically second nature and has resulted in great equipment longevity, especially where the transmission is concerned. I say, forget double clutching and just match revs, and get a cheap stick car to commute in and practice in. That way is becomes second nature. This begs the question......does anyone here do any left foot braking? I tried but this old dog found that to be a new trick.
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Old 12-05-2015, 09:38 AM   #14
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Any braking that doesn't require a downshift, (or ALL of it when I'm in a paddle shift or automatic trans car), is done with my left foot. On and off the track.

It's another practice that simply takes time to perfect. The only way to get good at it is to be terrible at it for a while...
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Old 12-05-2015, 10:21 AM   #15
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No one posted the video of Fast and Furious saying double clutching not granny shifting like you should be? Guess we are all adults here
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Old 12-06-2015, 08:02 AM   #16
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Quote:
Originally Posted by Coop View Post
..does anyone here do any left foot braking? I tried but this old dog found that to be a new trick.
Yea, a lot but, 10 years of running karts at T-Hill, Sears was good practice to get the modulation thing right.

It did translate into running the Formula Ford especially in corners where no downshift is required. T1 at T-Hill - flat footing the gas and just brushing the brake maintains the flow thru the air pump (engine) and it's a noticeable difference on corner exit. Use it in a couple other turns there too.

It worked well in The FF too because it had both a right foot and a left foot brake pedal.
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Old 03-01-2016, 07:28 PM   #17
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Default Double-clutching

Interesting and useful discussion. Cory nailed it. Coming from a generation of cars with either no synchros on some gears, or worn synchros; I learned to double-clutch early on and it's totally second nature.

It's so ingrained in me now that I'd have to relearn to downshift. While it's true that synchros do act as a clutch to match gear speeds, it's not like a constant-mesh motorcycle transmission. Double-clutch down-shifting reduces wear on the synchros - in addition to the need for the inherent rev-matching requirement for performance driving.

If you're racing with dog-type synchros, or a pro driving a car for one race and not caring about the wear; then double-clutching is surperflous. If you are like me, where I don't really want to have to rebuild my transmission; it's not only enjoyable, but allows the transmission to last the life of the car.

And, with respect to down-shifting, I don't understand how double-clutching is slower; since you only do it under braking.
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